Method for manufacture of a semiconductor wafer suitable for the manufacture of an soi substrate, and soi substrate wafer thus obtained

ABSTRACT

A semiconductor wafer suitable for fabricating an SOI substrate is provided by: producing a first layer of polycrystalline semiconductor on a top side of a semiconductor carrier; then forming an interface zone on a top side of the first layer, wherein the interface zone has a structure different from a crystal structure of the first layer; and then producing a second layer of polycrystalline semiconductor on the interface zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 filing from PCT/EP2015/056719 filed Mar. 27,2015, which claims priority to French Application for Patent No. 1452845filed Mar. 31, 2014, the disclosures of which are hereby incorporated byreference.

TECHNICAL FIELD

The invention relates to the field of electronics and more precisely tothe field of semiconductor substrates allowing electronic circuits to befabricated, especially electronic circuits dedicated to radio-frequencyapplications.

In particular, methods of implementation and embodiments of theinvention relate to a process for producing a silicon-on-insulator (SOI)structure, and to such a structure.

BACKGROUND

An SOI substrate conventionally comprises a semiconductor film (or topsemiconductor layer) located on top of a buried insulating layer(generally of silicon oxide), which is commonly designated by theacronym BOX (for Buried OXide), itself located on top of a carriersubstrate, a bulk substrate for example.

High-performance radio-frequency (RF) integrated circuits are generallyfabricated on what are referred to as high-resistivity (HR) p-dopedsemiconductor substrates, i.e. substrates with a resistivity typicallyhigher than 1 kohm·cm, so as to limit losses in the substrate andcrosstalk between neighboring components due to conduction.

Furthermore, it is common to use silicon-on-insulator (SOI) HRsubstrates. The passive or active components formed in and on the topsemiconductor layer are then insulated from the bulk substrate by theburied oxide layer.

However, it has been observed that although using such substratesdecreases losses related to the substrate, it does not completelyprevent them. Specifically, stationary positive electric charges areinevitably present in the BOX due to the fabrication process of thisoxide layer. Said charges are responsible for the accumulation in thesubstrate, in the vicinity of the BOX, of mobile electric charges(electrons) that are able to form a conductive channel. Thus, even if HRsubstrates are used, parasitic surface conduction in the substrate leadsto Joule losses as a result of eddy currents.

Furthermore, the insulated silicon/BOX/substrate stack behaves as ametal oxide semiconductor (MOS) capacitor. To a first approximation, thethickness of the inversion layer associated with this MOS capacitor isconsidered to vary inversely with the square root of the dopantconcentration of the substrate. Therefore, it will be understood thatthis thickness is correspondingly larger when the substrate is an HRsubstrate, i.e. weakly doped. The capacitance of this capacitor has theproperty of being modulated by the electric potential applied to thecomponents formed in or on the insulated silicon film, opposite andabove the BOX. This modulated parasitic capacitance is responsible forharmonic distortions and crosstalk affecting the RF components ofintegrated circuits in a way that is detrimental to the performance ofthese components. Such distortion may in particular be quantified by thethird-order intercept point (abbreviated TOIP or IP3) method.

In order to mitigate this effect, it is known to use more complexcarrier substrate structures, incorporating a specific region separatingthe single-crystal portion of the carrier substrate and the BOX so as tocreate, near the BOX, a high density of surface states capable oftrapping free carriers. This trapping of free carriers by surface statesgreatly decreases the effect of parasitic surface conduction.Furthermore it leads to pinning of the Fermi level in the semiconductorsat the substrate/BOX interface, thereby making the capacitance of theparasitic MOS capacitor largely independent of the electric potentialapplied to the components formed opposite, above the BOX, and thuslimiting harmonic distortions. Such substrates are qualified“trap-rich”.

The document “SOT technology: An Opportunity for RF Designers,” Journalof Telecommunications and Information Technology, 04/2009 (incorporatedby reference) describes a particularly effective method for forming atrap-rich layer under the BOX of an SOI substrate, consisting in forminga carrier substrate comprising a polysilicon layer inserted between thesingle-crystal portion of the carrier substrate and the BOX.Specifically, discontinuities in the crystal structure at the grainboundaries of the polysilicon act as charge traps. Such a layer may beformed by depositing on the single-crystal substrate, before the BOX hasbeen formed, a polysilicon or amorphous silicon layer, by way oflow-pressure chemical vapor deposition (LPCVD) or plasma-enhancedchemical vapor deposition (PECVD), this deposition operation beingfollowed by a stabilization and/or recrystallization anneal. To ensurethat a polycrystalline layer is indeed obtained rather than simpleepitaxy from the single-crystal substrate, it is necessary to form onthe latter, before the silicon is deposited, an interface layer thatblocks epitaxial growth. A silicon oxide layer of less than 1 nm inthickness, obtained by chemical or thermal oxidation, is sufficient forthis purpose.

In order to increase the density of traps, it is desirable to increasethe density of grain boundaries at the polysilicon/BOX interface, i.e.to decrease the average width of the grains at this interface.Unfortunately, the inventors have observed that the growth of thepolysilicon obtained by CVD is of the ‘divergent’ pseudo-columnar type,i.e. the grains have substantially the shape of inverted cones thatwiden between the bottom of the layer and its top. The average width ofthe grains is therefore maximal at the top of the polysilicon layer,i.e. at the polysilicon/BOX interface, and therefore the density oftraps per unit area at this interface is not optimal.

However, in order to ensure the single-crystal portion of the carriersubstrate is separated from the BOX and the components produced on thethin layer located on top of the BOX by a large enough distance, thepolysilicon layer must be at least 1000 nm to 5000 nm in thickness, and,as a result, the average width of the polysilicon grains at thepolysilicon/BOX interface is then about 200 nm to 1000 nm. For RFcomponents in 90 nm or 65 nm technology or lower, this therefore meansthat the average width of the polysilicon grains at the interface withthe BOX is larger than the lateral dimension of the elementarytransistors above the BOX. Thus, depending on the location of thecomponents above the BOX, transistors for example, said components willrandomly either be plumb with a single grain or a plurality of grains.This leads to a dispersion in the electrical parameters of thetransistors since the density of traps under a transistor depends on itsposition with respect to the grains.

The inventors have observed that forming the polysilicon layer bycontrolled recrystallization of an amorphous silicon layer does notsubstantially decrease the average size of the grains of polysilicon atthe polysilicon/BOX interface. Specifically, a minimum thermal budget isrequired to sufficiently stabilize a multilayer substrate and to make itthermomechanically stable enough to be able to used as a startingsubstrate for the fabrication of integrated circuits. This thermalbudget leads to a polysilicon layer the grain size of which is equal toor larger than that obtained by depositing a polysilicon layer directlyon the bulk substrate.

There is therefore a need for a method for fabricating an SOI substratethat remedies all or some of the above drawbacks.

SUMMARY

According to one method of implementation and embodiment, a method isprovided for producing an SOI substrate especially allowing the trappingof electrical charges at the interface between the electricallyinsulating layer (BOX) and the carrier substrate of the SOI substrate tobe improved.

According to one aspect, a process is provided for producing asilicon-on-insulator substrate including producing a carrier substrateand producing a semiconductor film located on top of a buried insulatinglayer itself located on top of the carrier substrate.

According to this aspect, the production of the carrier substratecomprises producing, on one side of a semiconductor carrier, a stacklocated between said semiconductor carrier and said buried insulatinglayer and including at least one initial structure.

The production of said initial structure includes the followingsuccessive steps:

-   -   producing, on said side of said semiconductor carrier, a first        layer of polycrystalline semiconductor;    -   forming an interface zone on the top side of said first layer,        said interface zone having a structure different from the        crystal structure of said first layer; and    -   producing, on said interface zone, a second layer of        polycrystalline semiconductor.

These stacked interface zones and layers are therefore advantageouslyformed in sucession starting from and using one and the same carrier,namely starting from and using said same semiconductor carrier.

According to one method of implementation, the formation of theinterface zone comprises exposing the top side of the first layer to anoxidizing environment in order to oxidize said top side.

As a variant, the formation of the interface zone comprises amorphizingthe top side of said first layer.

The production of said stack may furthermore comprise producing at leastone additional structure on said second layer, this including producinga new interface zone surmounted by a new layer of polycrystallinesemiconductor, so as to form a stack of a plurality of layers ofpolycrystalline semiconductor, the layers of polycrystallinesemiconductor respectively being separated by interface zones having astructure different from the crystal structure of said layers ofpolycrystalline semiconductor.

According to one method of implementation, the last layer ofpolycrystalline semiconductor of said stack has a thickness smaller than20% of the total thickness of the layers of polycrystallinesemiconductor of said stack.

According to another aspect, a silicon-on-insulator substrate isprovided comprising a semiconductor film located on top of a buriedinsulating layer itself located on top of a carrier substrate.

According to this aspect, the carrier substrate includes a semiconductorcarrier and a stack located between one side of the semiconductorcarrier and said buried insulating layer.

Said stack includes at least one initial structure including a firstlayer of polycrystalline semiconductor making contact with said side ofthe semiconductor carrier and a second layer of polycrystallinesemiconductor, said second layer being separated from the first layer byan interface zone having a structure different from the crystalstructure of said first layer.

The thickness of the second layer may be comprised between 100 nm and300 nm.

According to one embodiment, the substrate may furthermore comprise atleast one additional structure on said second layer, including a newinterface zone surmounted by a new layer of polycrystallinesemiconductor, so as to form a stack of a plurality of layers ofpolycrystalline semiconductor, said layers respectively being separatedby interface zones having a structure different from the crystalstructure of said layers of polycrystalline material.

The last layer of polycrystalline semiconductor of the stack may have athickness smaller than 20% of the total thickness of the layers ofpolycrystalline semiconductor of the stack.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain aspects of the invention will be better understood on readingthe following description, which is given merely by way of example, andwith reference to the appended figures, in which:

FIGS. 1 to 4 and 6 are simplified schematics of a partial side view of asemiconductor wafer forming a carrier substrate of an SOI substrateduring its fabrication; and

FIG. 5 is a simplified schematic of a side view of a variant embodimentof such a semiconductor wafer.

DETAILED DESCRIPTION

Of course, to make the figures easier to understand, the variouselements shown, and in particular the layers making up the semiconductorwafers, have been drawn schematically and the proportions of thesevarious elements may differ from their actual proportions.

The description below in particular relates to an example of a processfor producing a semiconductor wafer forming a carrier substrate of anSOI substrate, and certain variant embodiments, and describes certaineffects and advantages resulting from the structure of such a wafer.

In order not to confuse the reader, only steps that are important to theproduction of such a wafer will be detailed, it being understood that aperson skilled in the art will implement conventional processes relatingto preparation, cleaning and finishing operations inter alia, without itbeing necessary to describe them in detail here.

The process starts with the preparation of a semiconductor base carrier,also referred to as a wafer, such as illustrated in FIG. 1. This carrier1 may be produced from any semiconductor, and in particular is based onsilicon. Thus, in the rest of the description, the base semiconductorwill be described as being silicon, it being understood that it could bereplaced by any other known semiconductor, and in particular germaniumor gallium arsenide.

The embodiment described here is particularly advantageous forhigh-resistivity semiconductor carriers, i.e. carriers with relativelylow doping, that typically have a resistivity higher than 1 kohm·cm.This type of substrate is particularly appreciated for the production ofradio-frequency components.

Thus, in a first step, the top side 2 of the substrate 1 is prepared inorder to allow a polysilicon layer to be deposited. Next, as illustratedin FIG. 2, a first polysilicon layer 4 is formed on the top side 2 ofthe semiconductor carrier 1. More precisely, this first layer 4 isdeposited on the top side 2 of the carrier 1 by a chemical vapordeposition technique, so as to form a layer 4 the thickness e₄ of whichis typically comprised between 500 nanometers and 5000 nanometers andpreferably between 1000 nanometers and 2000 nanometers (the thickness e₄is defined in a direction normal to the interface between two layers).Optionally, after it has been deposited, the first layer 4 may besubjected to a heat treatment appropriate for its thickness and itsnature, so as to stabilize its structure by crystallizing it. On accountof the surface finish of the substrate layer 1 before this deposition,the silicon atoms organize into local crystal arrangements, providing adivergent pseudo-columnar structure, with grains 5, 6 of a substantiallypyramidal shape that widens toward the top, as schematically illustratedin FIG. 2. Heat treatment operations may modify the grains and thegrouping of certain grains into grains of larger size.

Of course, other deposition techniques may be employed to form thisfirst layer, for example low-pressure chemical vapor deposition (LPCVD),or more generally any type of known deposition technique allowingpolysilicon layers to be produced.

In a subsequent step, illustrated in FIG. 3, a surface treatment may becarried out to create an interface zone 12 on which a new polysiliconlayer will be deposited.

Specifically, the top side 7 of the first polysilicon layer 4 is dividedinto various regions 8, 9 each belonging to a separate grain, eachregion having a regular crystal structure. Silicon deposited directly onthis side 7 would grow epitaxially and thus continue the growth of thegrains of the underlying layer. However, one of the stated objectives isto increase the density of the boundaries 11 between grains, andtherefore to preserve small grains. To prevent epitaxial growth, thesecond step comprises forming an interface zone on the surface 7 of thefirst polysilicon layer 4, thereby allowing a new polysilicon layer tobe grown without epitaxial regrowth and therefore with a maximum ofseparate grains.

To do this, various techniques may be employed. It is thus possible toexpose the top side 7 of the first layer 4 of polysilicon to anenvironment comprising oxidizing species. This oxidation may be obtainedsimply by exposing the wafer, still sufficiently hot following thedeposition of the first polysilicon layer 4, to the ambient atmosphere,or to a controlled atmosphere including a percentage of oxygen. Naturaloxidation effects create, on the surface 7 of the polysilicon layer 4, alayer 12 of very small thickness e₁₂ of about the order of a nanometer,which layer has a structure different from the crystal structure of thepolysilicon grains (the thickness e₁₂ of the layer shown in FIG. 3 is ofcourse exaggerated in order to make it possible to see). This oxidationmay also be achieved via a controlled thermal oxidation such as a rapidthermal oxidation (RTO) i.e. oxidation the rate of which is increased byheating the wafer, typically to between 600 and 900° C., in the presenceof an oxidizing atmosphere.

It is also possible to obtain this oxidation by carrying out a chemicaloxidizing treatment in a conventional way.

Among other possible ways of producing the interface zone 12, mentionmay be made of operations for implanting non-dopant ionic species, suchas argon, germanium or any other suitable heavy ion. Such ionbombardment allows the surface 7 of the polycrystalline layer 4 to be atleast partially amorphized, i.e. the crystal arrangement on the surfaceof the grains to be broken, thereby making it possible to avoidepitaxial growth when a new polysilicon layer is deposited.

In a subsequent step, illustrated in FIG. 4, a second layer 14 ofsilicon is deposited on the interface zone 12. This second layer 14 mayadvantageously be produced in an identical way to that described for thelayer 4, thus allowing a second layer 14 of a thickness e₁₄ to beproduced, e₁₄ typically being about a few tens of nanometers to a fewhundred nanometers and preferably between about 100 and 300 nm.

Because the layer 14 is grown on an interface zone that is amorphous, orunstructured as explained above, the crystals of the second polysiliconlayer do not form epitaxially on the crystals of the first layer 4. Inother words, the top side of the first layer 4 is unstructured and hencethe crystals of the first layer 4 are not extended and enlarged in thesecond layer. Thus, polysilicon grains 15, 16 that are small in size areobserved to appear in the region closest to the interface zone 12. Thisresults in an overall increase in the concentration per unit volume ofgrain boundaries, i.e. of borders 17 between the various polysilicongrains, in the entire region located above the single-crystal substratelayer 1 (semiconductor carrier). This high concentration of grainboundaries per unit volume allows the density of charge traps to besubstantially increased in the region that will be located under theBOX. It will be noted here that the interface 12 has a function similarto the aforementioned interface 2.

At this stage, as illustrated in FIG. 4, a carrier substrate of an SOIsubstrate has been formed, said substrate including a semiconductorcarrier 1 and a stack located on the side 2 of the semiconductor carrier1 and intended, as will be seen with reference to FIG. 6, to be locatedbetween the side 2 of the carrier and the buried insulating layer (BOX)of the SOI substrate.

Said stack here includes an initial structure including the first layer4 of polycrystalline semiconductor making contact with said side 2 ofthe semiconductor carrier and the second layer 14 of polycrystallinesemiconductor, said second layer 14 being separated from the first layer4 by the interface zone 12 having a different structure from the crystalstructure of said first layer.

As a variant, it is possible, as illustrated in FIG. 5, to repeat theoperation of depositing an additional polysilicon layer 24 on a newinterface zone 22 that is functionally analogous to the interface zone12.

As may be seen in this FIG. 5, the additional structure including thenew interface zone 22 surmounted by the new layer 24 of polycrystallinesemiconductor leads to the formation of a stack of a plurality of layers4, 14, 24 of polycrystalline semiconductor respectively separated byinterface zones 12, 22 having a different structure from the crystalstructure of said layers 4, 14, 24 of polycrystalline semiconductor.

Of course, the number of superposed polysilicon layers may be multipliedif it is desired to further increase the concentration of grainboundaries per unit volume. It will be noted that the smaller thethickness of each of these polysilicon zones, the greater the density ofgrain boundaries in the layer, on account of the inverted pyramidalgrowth of the polysilicon.

It is also conceivable to select the individual thickness of each of thepolysilicon layers to ensure a preset variation of the concentration ofgrain boundaries per unit volume as a function of distance from the BOX.In one preferred embodiment, the thickness of the last polysilicon layerforming the stack is smaller than 20% of the total thickness of saidstack of polysilicon layers.

Next, and as illustrated in FIG. 6, the wafer (carrier substrate) 1, 4,12, 14 thus obtained may be finished by known techniques with a view toproducing an SOI substrate 2″ comprising the semiconductor film 40located on top of the buried insulated layer (BOX) 36. Thus, inparticular, as the roughness of the thick polysilicon deposits may beincompatible with the direct bonding process employed to bond theinsulated silicon film, the stack will advantageously be planarized, forexample by chemical-mechanical polishing CMP. Mention may also be made,by way of example, of the possibility of producing an oxide layer 36 onthe last characteristic polysilicon layer 14, which oxide layer willthen be bonded by direct bonding to the single-crystal film 40 that willform the thin layer (semiconductor film) of the SOI substrate. It isalso possible for the insulating layer 36 (BOX) of the SOI substrate tobe produced on a separate wafer, then bonded by direct bonding with thethin single-crystal layer to an analogous oxide layer produced on thelast polysilicon layer. Other techniques for completing the SOIsubstrate may be applied to the base structure (carrier substrate) 1, 4,12, 14 described above, comprising a single-crystal substrate on whichat least two polysilicon layers are stacked.

It will be apparent from the above that an SOI substrate produced asdescribed above allows the effect of parasitic surface conduction in thesingle-crystal substrate, in the zone located under the BOX, to bedecreased, including when the substrate is a high-resistivity substrate.Specifically, the high concentration of grain boundaries allows theeffective resistivity of the semiconductor to be increased. Furthermore,the high concentration of grain boundaries also allows the potentialmodulation of the capacitance of the parasitic capacitor created by theBOX and the underlying substrate, with respect to the bias applied tothe electronic components formed on the SOI substrate, to be limited. Ina complementary manner, because of the decrease in the size of thegrains in the region located under the BOX, the number of grains presentunder the various components produced in the thin layer (semiconductorfilm) of the SOI substrate is increased, and the risk of dispersion fromone component to another is limited.

1. Process for producing a silicon-on-insulator substrate, including:producing a carrier substrate, and producing a semiconductor filmlocated on top of a buried insulating layer itself located on top of thecarrier substrate, wherein producing the carrier substrate comprisesproducing, on one side of a semiconductor carrier, a stack locatedbetween said semiconductor carrier and said buried insulating layer andincluding at least one initial structure, wherein producing said atleast one initial structure comprises the following successive steps:producing, on said one side of said semiconductor carrier, a first layerof polycrystalline semiconductor; forming an interface zone on a topside of said first layer, said interface zone having a structuredifferent from a crystal structure of said first layer; and producing,on said interface zone, a second layer of polycrystalline semiconductor.2. Process according to claim 1, wherein forming the interface zonecomprises exposing the top side of the first layer to an oxidizingenvironment in order to oxidize said top side of the first layer. 3.Process according to claim 1, wherein forming the interface zonecomprises amorphizing the top side of said first layer.
 4. Processaccording to claim 1, wherein producing said stack further comprisesproducing at least one additional structure on said second layer byproducing a new interface zone surmounted by a new layer ofpolycrystalline semiconductor, so as to form a stack of a plurality oflayers of polycrystalline semiconductor, wherein layers of the stack ofthe plurality of layers of polycrystalline semiconductor arerespectively separated by interface zones having a structure differentfrom the crystal structure of said layers of polycrystallinesemiconductor.
 5. Process according to claim 4, wherein a last layer ofplurality of layer of polycrystalline semiconductor of said stack has athickness smaller than 20% of a total thickness of preceding layers ofpolycrystalline semiconductor of said stack.
 6. Silicon-on-insulatorsubstrate comprising: a semiconductor film located on top of a buriedinsulating layer itself located on top of a carrier substrate, whereinthe carrier substrate includes: a semiconductor carrier and a stacklocated between one side of the semiconductor carrier and said buriedinsulating and including at least one initial structure including afirst layer of polycrystalline semiconductor making contact with saidone side of the semiconductor carrier and a second layer ofpolycrystalline semiconductor, said second layer being separated fromthe first layer by an interface zone having a structure different fromthe crystal structure of said first layer.
 7. Substrate according toclaim 6, further comprising at least one additional structure on saidsecond layer, including a new interface zone surmounted by a new layerof polycrystalline semiconductor, so as to form a stack of a pluralityof layers of polycrystalline semiconductor, wherein layers of said stackare respectively separated by interface zones having a structuredifferent from a crystal structure of said plurality of layers ofpolycrystalline material of said stack.
 8. Substrate according to claim7, wherein a last layer of polycrystalline semiconductor of the stackhas a thickness smaller than 20% of a total thickness of precedinglayers of polycrystalline semiconductor of the stack.
 9. SOI substrateaccording to claim 6, wherein a thickness of the second layer is between100 nm and 300 nm.
 10. A process for producing an SOI substrate,comprising: producing a first layer of polycrystalline semiconductorabove a semiconductor carrier, said first layer having a divergentpseudo-columnar crystal arrangement with a grouping of grains having afirst size; then forming an interface zone on a top side of said firstlayer, said interface zone having a structure different from a crystalstructure of said first layer; then producing a second layer ofpolycrystalline semiconductor on said interface zone, said second layerhaving a divergent pseudo-columnar crystal arrangement with a groupingof grains having a second size smaller than the first size.
 11. Theprocess of claim 10, wherein the first layer has a thickness between 500nm and 5000 nm and the second layer has a thickness between 100 nm and300 nm.
 12. The process of claim 10, wherein forming the interface zonecomprises one of oxidizing or amorphizing the top size of said firstlayer.
 13. An SOI substrate, comprising: a semiconductor carrier havinga top side; a first layer of polycrystalline semiconductor above the topside of the semiconductor carrier, said first layer having a divergentpseudo-columnar crystal arrangement with a grouping of grains having afirst size; an interface zone making contact with a top side of thefirst layer, said interface zone having a structure different from acrystal structure of said first layer; and a second layer ofpolycrystalline semiconductor on said interface zone, said second layerhaving a divergent pseudo-columnar crystal arrangement with a groupingof grains having a second size smaller than the first size.
 14. Thesubstrate of claim 13, wherein the first layer has a thickness between500 nm and 500 nm and the second layer has a thickness between 100 nmand 300 nm.
 15. The substrate of claim 13, wherein the interface zonecomprises one of an oxidized region or an amorphized region of saidfirst layer.